meson: add description to options

This will be useful in the future to generate configure
command line parsing from meson_options.txt.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

meson_options.txt

@@ -2,12 +2,23 @@ option('qemu_suffix', type : 'string', value: 'qemu',
        description: 'Suffix for QEMU data/modules/config directories (can be empty)')
 option('docdir', type : 'string', value : 'doc',
        description: 'Base directory for documentation installation (can be empty)')
-option('gettext', type : 'boolean', value : true)
-option('sdl', type : 'feature', value : 'auto')
-option('sdl_image', type : 'feature', value : 'auto')
-option('u2f', type : 'feature', value : 'auto')
-option('vnc', type : 'feature', value : 'enabled')
-option('vnc_jpeg', type : 'feature', value : 'auto')
-option('vnc_png', type : 'feature', value : 'auto')
-option('vnc_sasl', type : 'feature', value : 'auto')
-option('xkbcommon', type : 'feature', value : 'auto')
+
+option('gettext', type : 'boolean', value : true,
+       description: 'Localization of the GTK+ user interface')
+
+option('sdl', type : 'feature', value : 'auto',
+       description: 'SDL user interface')
+option('sdl_image', type : 'feature', value : 'auto',
+       description: 'SDL Image support for icons')
+option('u2f', type : 'feature', value : 'auto',
+       description: 'U2F emulation support')
+option('vnc', type : 'feature', value : 'enabled',
+       description: 'VNC server')
+option('vnc_jpeg', type : 'feature', value : 'auto',
+       description: 'JPEG lossy compression for VNC server')
+option('vnc_png', type : 'feature', value : 'auto',
+       description: 'PNG compression for VNC server')
+option('vnc_sasl', type : 'feature', value : 'auto',
+       description: 'SASL authentication for VNC server')
+option('xkbcommon', type : 'feature', value : 'auto',
+       description: 'xkbcommon support')